2,3,5-Trimethylhydroquinone diesters are not only valuable intermediates in the synthesis of vitamin E and other pharmaceutically active chromane compounds, but are also widely used as antioxidants.
The formation of 2,3,5-trimethylhydroquinone diesters from KIP in the presence of a strongly acidic catalyst and an acylating agent such as carboxylic anhydrides, acyl halides or enol esters is known and has been described in several patents (for example DE 26 46 172 C2, EP 808 815 A2 and EP 0 850 910 A1).
It is furthermore known that while this aromatization may indeed be performed at up to 100% conversion, primarily depending upon the catalyst, selectivity is between 80 and 95%. The secondary product which is mainly formed is the isomeric 3,4,5-trimethylpyrocatechol diester. This secondary reaction, which always occurs, is described in EP 0 850 912 A1.
Isolating the 2,3,5-trimethylhydroquinone diester from the reaction solution and purifying it is complex and entails considerable losses. In examples using dissolved catalysts, the mixture of 2,3,5-trimethylhydroquinone diester and the 3,4,5-trimethylpyrocatechol diester is precipitated by addition of water or basic aqueous solutions and purified by recrystallization from organic solvents. Recovering the catalyst from the filtrate is complex and the yield is greatly reduced on recrystallization. The acylating agent, which is introduced into the aromatization reaction in excess, hydrolyzes and is unusable for subsequent reactions.
When undissolved catalysts are used, such as strongly acidic ion exchangers or H-Y zeolites, separation and reuse of the catalyst is simple, but the isolation and high-loss purification of the 2,3,5-trimethylhydroquinone diester are no different from the reaction solutions using dissolved catalysts.
Due to the low isolation yield and the large quantities of waste, as an industrial production process, this method is associated with serious economic and environmental disadvantages.